In Praise of Usability Testing
http://www.stcboston.org/archives/articles/usability2.shtml

Hosier, William J.
Boston Broadside
1991

Abstract:

Documentation usability testing has been with us at least since 1983. But, to all intents and purposes, it seems that there is still very little testing being done. Of all the documentation quality assurance activities that we can carry out, usability testing represents one of the best values in the process chain. Without devaluing any of the other quality assurance functions, it is safe to say that money spent on usability testing is extremely well spent.